174 



EVAPORATION. 



he called a cryophorus. This instrument consisted of a glass tube, A B, fig. 1, 

 furnished with two bulbs, C D, placed on short branches at right angles to it. 



Fig. 1. 



A small quantity of water is introduced through a short tube, which proceeds 

 from the bottom of the bulb D at O. It is boiled in C until the space above C, 

 and tube A B, and the bulb D, is completely filled with aqueous vapor to the 

 exclusion of atmospheric air. The tube is then closed by melting it with a 

 blowpipe, so that the interior of the apparatus now contains nothing but water. 

 When the instrument cools, the vapor is condensed, and such a vapor only 

 subsists in the instrument as corresponds to the temperature of the water in C. 

 If the bulb D be now surrounded by a freezing mixture, or exposed to any in- 

 tense cold, the vapor produced from the water in C will be condensed in it, so 

 that the space above the water in C, and in the tube A B, will be constantly 

 prevented from attaining the state of saturation. The evaporation will then be 

 continued, and the latent heat of the steam must be chiefly derived from the 

 sensible heat of the water remaining in C. The temperature, therefore, of this 

 water will be rapidly depressed until it reaches the freezing point, when it will 

 be solidified. 



When an ink bottle has a large mouth, the surface of the liquid in it will be 

 exposed to a rapid evaporation ; and, as this evaporation affects only the aque- 

 ous part of the liquid, the effect will be, that the ink will first become thick, 

 and, if exposed a longer time, the whole of the liquid portion of it will pass off, 

 and nothing but the hard coloring matter will remain. If, however, the mouth 

 of the bottle be contracted to a small aperture, sufficient to receive a pen, the 

 rate of evaporation will be considerably diminished ; for, although the surface 

 of ink in the bottle may be large, yet the evaporation having, in the first in- 

 stance, saturated the space between the surface of the ink and the mouth of the 

 bottle, no farther evaporation could take place, if that mouth were stopped ; but, 

 if it be opened, then a portion of the vapor, contained in the bottle above the 

 surface of the liquid, will escape from it into the strata of air immediately above ; 

 but this portion will be less in proportion as the mouth of the bottle is small. 

 It will, therefore, be found that ink will be less liable to thicken in ink-bottles 

 having a small aperture than in those which have a large aperture ; but the 

 thickening of ink may be altogether avoided by the use of ink-bottles which, 

 while they are capable of containing a considerable quantity of ink, expose a 

 very small surface to evaporation. Such bottles are constructed like bird-cage 

 fountains. A B, fig. 2, is a glass bottle, completely closed at the top, and hav- 



Fig. 2. 



ing a tube, C, proceeding laterally from the bottom turned upward, where there 

 is a small mouth large enough to receive a pen. The bottle is filled by inch- 



